Integrated circuits are basically formed of a plurality of layers, where different features are formed in each of the various layers. The features in each of the various layers are typically formed using a photolithographic process. As a part of the process, a photolithographic mask is prepared with the desired image formed in the mask. The substrate on which the integrated circuit is to be formed is coated with a photosensitive material called photoresist, and the photoresist is exposed with a light that is passed through the mask. Thus, the image present in the mask is projected onto the photoresist coated substrate, thereby exposing portions of the photoresist to the light, and masking other portions of the photoresist from the light.
Depending on the type of photoresist used, either negative or positive, those portions of the photoresist that are exposed to the light either remain after the photoresist is developed, or are washed away during the developing process. After a hard bake to drive out solvents from the remaining photoresist, the patterned layer on the substrate is processed in some manner, such as being etched, and the photoresist is removed. A new layer is then deposited or otherwise formed, and the process repeats itself until wafer form processing of the integrated circuit is substantially completed.
There has been a tremendous effort throughout the history of integrated circuit technology to continually find ways to reduce the size of the devices and structures within the monolithic integrated circuits so fabricated. Many problems inherent with the shrinking geometries of integrated circuits have been identified and overcome over the years of such development.
For example, one problem has to do with the imaging process described above. As device size has become increasingly smaller, so too has the size of the mask image used to create the device become increasingly smaller. The size of the image on the mask has become so small, that the images are difficult to resolve properly because the feature size is very close to (or even smaller than) the wavelength of the light used to image the features. These problems are generally described as proximity effects.
What is needed, therefore, is system by which very small devices can be properly imaged in a photoresist coated substrate and proximity effects can be reduced.